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One-pot hydrothermal preparation of graphene sponge for the removal of oils and organic solvents
Applied Surface Science 362 (2016) 56–62
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
One-pot hydrothermal preparation of graphene sponge for the removal of oils and organic solvents Ruihan Wu, Baowei Yu, Xiaoyang Liu, Hongliang Li, Weixuan Wang, Lingyun Chen, Yitong Bai, Zhu Ming, Sheng-Tao Yang ∗ College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, China
a r t i c l e
i n f o
Article history: Received 24 September 2015 Received in revised form 20 November 2015 Accepted 22 November 2015 Keywords: Graphene sponge Adsorption Oil Organic solvent Water treatment Nanotechnology
a b s t r a c t Graphene sponge (GS) has found applications in oil removal due to the hydrophobic nature of graphene sheets. Current hydrothermal preparations of GS use toxic reducing reagents, which might cause environmental pollution. In this study, we reported that graphene oxide (GO) could be hydrothermally reduced by glucose to form GS for the adsorption of oils and various organic solvents. Graphene sheets were reduced by glucose during the hydrothermal treatment and formed 3D porous structure. GS efficiently adsorbed organic solvents and oils with competitive adsorption capacities. GS was able to treat pollutants in pure liquid form and also in the simulated seawater. GS could be easily regenerated by evaporating or burning. After 10 cycles, the adsorption capacity still retained 77% by evaporating and 87% by burning. The implication to the applications of GS in water remediation is discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Since its discovery, graphene has attracted tremendous interest and efforts due to the unique structure and fantastic properties [1–3]. Among the attractive applications, graphene has been used as adsorbents for various pollutants [4–6]. For instance, Yang and coworkers [7–11] developed graphene adsorbents for the adsorption of heavy metals, dyes and antibiotics. The adsorptions of pesticides, organic molecules and nonmetal ions on graphene adsorbents were also widely concerned [12–14]. During the evaluations, graphene adsorbents showed competitive performance to other adsorbents. More recently, graphene adsorbents found their applications in oil removal [15–20]. Due to the hydrophobic nature of oils, graphene should be in the reduced forms for oil removal. Porous graphene could accommodate more oils, thus, is the main form in the oil–water separation. There are several types of graphene for oil remediation. The first category is graphene sponge (GS). Hydrothermal methods are widely adopted in preparing GS that had impressive adsorption performance in oil adsorption [15,16]. Bi et al. [17] annealed graphene oxide (GO) aerogel to produce GS for oil removal. Pourmand et al. [18] reported that nanoporous
∗ Corresponding author. E-mail address: [email protected] (S.-T. Yang). http://dx.doi.org/10.1016/j.apsusc.2015.11.215 0169-4332/© 2015 Elsevier B.V. All rights reserved.
graphene prepared by chemical vapor deposition (CVD) had high adsorption capacities for oils. The second one is graphene amended polymer sponge [19–21]. Typically, polyurethane (PU) sponge was immersed in GO and then reduced to prepare graphene coated PU sponge for oil and organic solvent adsorption. The third one is graphene amended cotton. Ge et al. [22] coated cotton with graphene for oil/water separation. The fourth category is porous graphene composites. Graphene and other inorganic materials and/or polymers formed composites for oil adsorption [23–25]. Among the aforementioned categories, the preparation of GS is easier and more reproducible. In particular, hydrothermal reduction of GO is regarded as the most facile method to prepare GS for oil removal. However, current studies used reducing reagents that might cause environmental pollutions, such as thiourea and ammonia [15,16]. Therefore, the application of environmental friendly reducing reagents for the preparation of GS should be pursued. Herein, we reported the one-pot hydrothermal preparation of GS for oil and organic solvent removal by using glucose as the reducing reagent (Fig. 1). GS was characterized to confirm the porous structure and effective reduction. The adsorption of dodecane by GS in pure liquid form and in the simulated water was achieved. The adsorption capacities of diverse oils and organic solvents on GS were quantified. The regeneration of GS was performed by squeezing or burning. The implication to the applications of GS in water remediation is discussed.
R. Wu et al. / Applied Surface Science 362 (2016) 56–62
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Fig. 1. Schematic illustration of the hydrothermal preparation process of GS.
2. Materials and methods 2.1. Materials Graphite was purchased from Huayi Co., Shanghai, China. Glucose was obtained from Yili Fine Chemicals Co., Ltd., Beijing, China. Dodecane was purchased from Kelong Chemical Co., Chengdu, China. Rap oil and machine oil were obtained in local market. Gasoline was bought in local gas station. Crude oil was kindly provided by PetroChina Southwest Oil and Gasfield Company, China. Other chemicals were all of analytical grade.
To evaluate the adsorption ability of GS in seawater, 25 g of sodium chloride, 1.14 g of calcium chloride and 0.7 g of potassium chloride were added into 1 L of ionized water to simulate the seawater. Dodecane (2 g) was stained by Sudan red 5B and added to the simulated seawater. Then, GS (0.9 g) was used to adsorb the dodecane, where GS was drifted by a pipette. The complete removal was achieved after 5 min adsorption. To measure the adsorption capacity of GS for oils and organic solvents, GS was weighted (m0 ) and added to 20 mL of liquid (solvent or oil) in a beaker. After 24 h adsorption, GS was taken out and
2.2. Preparation of GS The preparation of GO was following modified Hummers method [26]. Briefly, graphite (3 g) was pre-oxidized by potassium persulfate (2.5 g) and phosphorus pentoxide (2.5 g) in 12 mL of concentrated sulfuric acid. The reaction was taken in water bath at 80 ◦ C for 4.5 h. The residue was collected and dried after pouring the mixture into 500 mL of deionized water. The pre-oxidized residue was added into 120 mL of concentrated sulfuric acid, and potassium permanganate (15 g) was added slowly under stirring. The mixture was stirred at 35 ◦ C for 2 h, and added into 250 mL of deionized water following by another 2 h stirring. Then 500 mL of deionized water was added and 20 mL of 30% hydrogen peroxide was added dropwise. The yellow product was filtered and washed with HCl aqueous solution (1 mol/L). The yellow solid was dialyzed for 3 days to give graphite oxide. Graphite oxide (10 mg/mL) was sonicated for 1 h to generate GO dispersion. GO dispersion (10 mg/mL) was mixed with glucose at the mass ratio of 1:1 and sonicated for 30 min. The mixture was sealed in 100 mL Teflon-lined autoclave and maintained at 160 ◦ C for 6 h. After cooling to room temperature, the black rod was washed with deionized water and lyophilized to produce GS. 2.3. Characterization GS samples were characterized by scanning electron microscopy (SEM, Quanta 200FEG, FEI, Netherlands), transmission electron microscopy (TEM, JEM-200CX, JEOL, Japan), X-ray photoelectron spectroscopy (XPS, Kratos, UK) and infrared spectrometer (IR, Magna-IR 750, Nicolet, USA). 2.4. Adsorption of oils and organic solvents To visualize the adsorption, dodecane (2 g) was stained by Sudan red 5B and then placed in a glass plate. After photographing, GS (0.4 g) was added. Photographs were taken at each time interval of 20 s. At the end of photographing, the plate was wiped with the GS to achieve the complete removal.
Fig. 2. SEM images of GS at lower (a) and higher (b) magnifications.
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weighted again (m). The adsorption capacity (q) was calculated as following equation:
GO sheets were reduced by glucose via hydrothermal treatment. During the reduction, graphene sheets lost the dispersibility in aqueous system and formed 3D porous structure. According to the SEM images (Fig. 2), GS had large pores (∼100 m) and the graphene sheets cross-linked. Graphene sheets were very large (∼200 m) when comparing to starting GO sheets (∼1 m). This suggesting that graphene sheets assembled to form larger sheets
and then cross-linked to produce 3D porous structure. In the contrast, the SEM image of GO was generally flat (Fig. S1a). Then we cut some GS samples and sonicated them in water for TEM investigation. Under TEM, the graphene sheets showed more wrinkles and folds (Fig. 3) than GO (Fig. S1b). No carbon particle was found on the graphene sheets of GS. This was consistent with the literature results [27–30]. For example, Ji et al. [27] hydrothermally reduced GO by sugars and no apparent particulate was presented on graphene sheets under TEM. Martin-Jimeno et al. [28] used very high glucose: GO ratio and observed the form of carbon layers on graphene sheets rather than spheres. In the contrast, pure glucose formed particles during the hydrothermal treatment. The porous structure and hydrophobic nature of GS made it floating on water easily. This might benefit the applications in oil adsorption, because oils usually float on water. XRD was adopted to reveal the crystalline of GS (Fig. 4). There was no peak found in the XRD spectrum of GO (Fig. S2a). After the reduction, a broad and weak peak was observed at 2 of 24.6◦ for GS. It migrated to lower 2 angles comparing to graphite (2 of 26.6◦ ). This suggested that the interlayer spacing of graphene layers (002) was larger in GS (0.36 nm) than in graphite (0.33 nm). In the IR spectrum, a broad band at 3436 cm−1 was assigned to COOH/ OH (Fig. 5a). The tiny peak at 2920 cm−1 corresponded to CH, which should be the remnant glucose residues. The peak at 1630 cm−1 was attributed to C O. The peak at 1030 cm−1 was
Fig. 3. TEM images of GS at lower (a) and higher (b) magnifications.
Fig. 4. XRD spectra of GS (a) and graphite (b).
q=
m − m0 m0
(1)
The recycling GS, upon the adsorption of dodecane, GS samples were squeezed and then dried by natural evaporation. The dried GS was used to adsorb dodecane again and the adsorption capacity was measured as described above. The recycling was repeated up to 10 cycles. To demonstrate the removal of dodecane by burning, a piece of GS was placed in a glass place and over dosed dodecane was added. The photographs were taken before and after firing. To measure the size of GS, a rule was place under the plate. 3. Results and discussion 3.1. Characterization of GS
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Fig. 5. IR (a), C1s XPS (b) and Raman (c) spectra of GS.
assigned to C O. The IR spectrum of GS was similar to that of GO. Such phenomenon was also observed in Ji et al.’s study [27]. The remnant oxygen was quantified by XPS. The atomic ratio of oxygen was 14.6%. The value was 33.5% for GO. Thus, efficient reduction was achieved according to XPS. The remnant oxygen of GS was reasonable and consistent with the literature results, in which chemical reduction was unable to remove all the oxygen atoms [31–33]. In
addition, the C1s XPS was recorded to reveal the chemical states of carbon atoms (Fig. 5b). The components of GS were C C (61.8%), C O (34.1%) and C O (4.13%). In the contrast, the components of GO were C C (43.7%), C O (49.3%) and C O (7.0%) (Fig. S2b). The Raman spectrum of GS showed similar intensities of G-band and D-band (Fig. 5c), while the intensity of G-band was weaker than that of D-band in GO (Fig. S2c).
Fig. 6. Adsorption of dodecane (dyed with Sudan red 5B) by GS. The photographs were taken before (a) and after adding of GS at time intervals of 20 s (b–h).
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Fig. 7. Adsorption of dodecane (dyed with Sudan red 5B) by GS in simulated seawater. (a) Dodecane in seawater, (b) at the beginning of adding GS, (c) at 8 min post the addition of GS. Table 1 Adsorption capacity of various adsorbents for oils and solvents. Adsorbent
Oil or solvent
Capacity (g/g)
Ref.
Corn stalk Polypropylene Modified polyurethane sponge Butyl rubber Graphene foam Carbon foam Graphene coated polyurethane Carbon nanofiber hydrogel Graphene sponge Polystyrene fibers Graphene-carbon nanotube GS
Gas oil Fuel oil Dodecane Crude oil Oil Wash oil Acetone Diesel oil Castor oil Silicon oil Toluene Crude oil
8 15.7 18 23 27.1 28.4 32 40 75 81.4 130 29.3
[38] [39] [40] [39] [24] [41] [20] [42] [15] [43] [44] This study
To demonstrate the removal of oil by GS, dodecane was stained by Sudan red 5B and placed in a glass plate for adsorption (Fig. 6a). Then, a piece of GS was added and photographed at time intervals of 20 s (Fig. 6b–f). The remnant dodecane was wiped to reach complete removal (Fig. 6g, h). The results clearly indicated that GS was capable for oil adsorption. Further, many oil leakages occur in sea, thus, we evaluated the adsorption of dodecane in simulated seawater (Fig. 7). After 8 min
adsorption, the dodecane was completely removed. The side view of the beaker was provided in Fig. S3. The adsorption was slower than in pure liquid form, because when the contacting dodecane was adsorbed, water would fill the gap between GS and dodecane. We used a sucker to drive GS toward dodecane and the adsorption was slowed down. During the adsorption experiment, GS floated steadily on the simulated seawater. It would benefit the practical applications, where GS can be collected easily after the adsorption. Then, the adsorption capacities of GS for various organic solvents and oils were quantified. As shown in Fig. 8, GS was capable in removal diverse pollutants. The adsorption capacities were in the range of 23–35 g/g. The ultrahigh adsorption performance suggested that the adsorption was more like accommodation of oils in the pores of GS. The adsorption mechanism should be that the pores were hydrophobic and the oils filled the pores. In particular, the adsorption capacity for crude oil was 29.3 g/g, indicating
Fig. 9. Contact angle of GS.
Fig. 10. Regeneration of GS after the adsorption of dodecane.
Fig. 8. Adsorption capacity of GS for organic solvents and oils.
3.2. Adsorption of oil and organic solvents on GS
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Fig. 11. Removal of dodecane from GS by burning. (a) Before firing, (b, c) during the burning, (d) after the burning.
that GS could be used for oil leakage treatment. The adsorption for machine oil had the highest capacity of 35.5 g/g. Due to the remnant oxygen atoms, GS was also capable for the adsorption of polar solvents, such as ethanol, acetone, and so on. This was due to the very weak hydrophilicity of GS. As shown in Fig. 9, the contact angle of GS sample was 83◦ , smaller than that of well reduced graphene [34]. It should be noted that although the adsorption capacities of our GS samples were lower than those of other GS from hydrothermal methods [15,16], the performance of our samples was higher than other high-performance adsorbents, such as alumina [35], polymers [36], and natural products [37]. The comparison was listed in Table 1. Considering the environmental friendly preparation [15,16], our GS would found practical applications in oil removal. For practical applications, the recycling of GS is very important for reducing the operating cost. The regeneration of GS was achieved by squeezing and evaporating. After 2 cycles, the adsorption capacity retained around 80% of the raw GS (Fig. 10). At 10 cycles, the adsorption capacity was 77% of the raw GS. In addition, GS could be recycled by burning on fire. As shown in Fig. 11, GS did not change much before and after the burning of dodecane. The Raman and IR spectra of GS before and after the burning did not change much, confirming the unchanged properties of GS during the burning (Fig. S4). The phenomenon should be attributed to the good thermo conductivity of GS. The regeneration by burning seemed to be even better than evaporating method. However, burning led to the waste of oil and would pollute the air. Therefore, we recommended the squeezing and evaporating to be better regenerating method.
4. Conclusion GS was prepared via one-pot hydrothermal reduction for oil removal, where environmental friendly glucose was used as the reducing reagent. Hydrophobic graphene sheets formed 3D porous structure that could accommodate organic solvents and oils. GS showed competitive adsorption capacities to other high performance adsorbents for oil removal. The facile regeneration of GS made it very promising in practical applications. It is hoped that our study would stimulate more interest on the graphene based environmental nanotechnologies.
Acknowledgments We acknowledge financial support from the Science and Technology Department of Sichuan Province (No. 2013FZ0060), the China Natural Science Foundation (No. 201307101), and the Innovation Scientific Research Program for Graduates in Southwest University for Nationalities (No. CX2016SZ024).
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
One-pot hydrothermal preparation of graphene sponge for the removal of oils and organic solvents Ruihan Wu, Baowei Yu, Xiaoyang Liu, Hongliang Li, Weixuan Wang, Lingyun Chen, Yitong Bai, Zhu Ming, Sheng-Tao Yang ∗ College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, China
a r t i c l e
i n f o
Article history: Received 24 September 2015 Received in revised form 20 November 2015 Accepted 22 November 2015 Keywords: Graphene sponge Adsorption Oil Organic solvent Water treatment Nanotechnology
a b s t r a c t Graphene sponge (GS) has found applications in oil removal due to the hydrophobic nature of graphene sheets. Current hydrothermal preparations of GS use toxic reducing reagents, which might cause environmental pollution. In this study, we reported that graphene oxide (GO) could be hydrothermally reduced by glucose to form GS for the adsorption of oils and various organic solvents. Graphene sheets were reduced by glucose during the hydrothermal treatment and formed 3D porous structure. GS efficiently adsorbed organic solvents and oils with competitive adsorption capacities. GS was able to treat pollutants in pure liquid form and also in the simulated seawater. GS could be easily regenerated by evaporating or burning. After 10 cycles, the adsorption capacity still retained 77% by evaporating and 87% by burning. The implication to the applications of GS in water remediation is discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Since its discovery, graphene has attracted tremendous interest and efforts due to the unique structure and fantastic properties [1–3]. Among the attractive applications, graphene has been used as adsorbents for various pollutants [4–6]. For instance, Yang and coworkers [7–11] developed graphene adsorbents for the adsorption of heavy metals, dyes and antibiotics. The adsorptions of pesticides, organic molecules and nonmetal ions on graphene adsorbents were also widely concerned [12–14]. During the evaluations, graphene adsorbents showed competitive performance to other adsorbents. More recently, graphene adsorbents found their applications in oil removal [15–20]. Due to the hydrophobic nature of oils, graphene should be in the reduced forms for oil removal. Porous graphene could accommodate more oils, thus, is the main form in the oil–water separation. There are several types of graphene for oil remediation. The first category is graphene sponge (GS). Hydrothermal methods are widely adopted in preparing GS that had impressive adsorption performance in oil adsorption [15,16]. Bi et al. [17] annealed graphene oxide (GO) aerogel to produce GS for oil removal. Pourmand et al. [18] reported that nanoporous
∗ Corresponding author. E-mail address: [email protected] (S.-T. Yang). http://dx.doi.org/10.1016/j.apsusc.2015.11.215 0169-4332/© 2015 Elsevier B.V. All rights reserved.
graphene prepared by chemical vapor deposition (CVD) had high adsorption capacities for oils. The second one is graphene amended polymer sponge [19–21]. Typically, polyurethane (PU) sponge was immersed in GO and then reduced to prepare graphene coated PU sponge for oil and organic solvent adsorption. The third one is graphene amended cotton. Ge et al. [22] coated cotton with graphene for oil/water separation. The fourth category is porous graphene composites. Graphene and other inorganic materials and/or polymers formed composites for oil adsorption [23–25]. Among the aforementioned categories, the preparation of GS is easier and more reproducible. In particular, hydrothermal reduction of GO is regarded as the most facile method to prepare GS for oil removal. However, current studies used reducing reagents that might cause environmental pollutions, such as thiourea and ammonia [15,16]. Therefore, the application of environmental friendly reducing reagents for the preparation of GS should be pursued. Herein, we reported the one-pot hydrothermal preparation of GS for oil and organic solvent removal by using glucose as the reducing reagent (Fig. 1). GS was characterized to confirm the porous structure and effective reduction. The adsorption of dodecane by GS in pure liquid form and in the simulated water was achieved. The adsorption capacities of diverse oils and organic solvents on GS were quantified. The regeneration of GS was performed by squeezing or burning. The implication to the applications of GS in water remediation is discussed.
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Fig. 1. Schematic illustration of the hydrothermal preparation process of GS.
2. Materials and methods 2.1. Materials Graphite was purchased from Huayi Co., Shanghai, China. Glucose was obtained from Yili Fine Chemicals Co., Ltd., Beijing, China. Dodecane was purchased from Kelong Chemical Co., Chengdu, China. Rap oil and machine oil were obtained in local market. Gasoline was bought in local gas station. Crude oil was kindly provided by PetroChina Southwest Oil and Gasfield Company, China. Other chemicals were all of analytical grade.
To evaluate the adsorption ability of GS in seawater, 25 g of sodium chloride, 1.14 g of calcium chloride and 0.7 g of potassium chloride were added into 1 L of ionized water to simulate the seawater. Dodecane (2 g) was stained by Sudan red 5B and added to the simulated seawater. Then, GS (0.9 g) was used to adsorb the dodecane, where GS was drifted by a pipette. The complete removal was achieved after 5 min adsorption. To measure the adsorption capacity of GS for oils and organic solvents, GS was weighted (m0 ) and added to 20 mL of liquid (solvent or oil) in a beaker. After 24 h adsorption, GS was taken out and
2.2. Preparation of GS The preparation of GO was following modified Hummers method [26]. Briefly, graphite (3 g) was pre-oxidized by potassium persulfate (2.5 g) and phosphorus pentoxide (2.5 g) in 12 mL of concentrated sulfuric acid. The reaction was taken in water bath at 80 ◦ C for 4.5 h. The residue was collected and dried after pouring the mixture into 500 mL of deionized water. The pre-oxidized residue was added into 120 mL of concentrated sulfuric acid, and potassium permanganate (15 g) was added slowly under stirring. The mixture was stirred at 35 ◦ C for 2 h, and added into 250 mL of deionized water following by another 2 h stirring. Then 500 mL of deionized water was added and 20 mL of 30% hydrogen peroxide was added dropwise. The yellow product was filtered and washed with HCl aqueous solution (1 mol/L). The yellow solid was dialyzed for 3 days to give graphite oxide. Graphite oxide (10 mg/mL) was sonicated for 1 h to generate GO dispersion. GO dispersion (10 mg/mL) was mixed with glucose at the mass ratio of 1:1 and sonicated for 30 min. The mixture was sealed in 100 mL Teflon-lined autoclave and maintained at 160 ◦ C for 6 h. After cooling to room temperature, the black rod was washed with deionized water and lyophilized to produce GS. 2.3. Characterization GS samples were characterized by scanning electron microscopy (SEM, Quanta 200FEG, FEI, Netherlands), transmission electron microscopy (TEM, JEM-200CX, JEOL, Japan), X-ray photoelectron spectroscopy (XPS, Kratos, UK) and infrared spectrometer (IR, Magna-IR 750, Nicolet, USA). 2.4. Adsorption of oils and organic solvents To visualize the adsorption, dodecane (2 g) was stained by Sudan red 5B and then placed in a glass plate. After photographing, GS (0.4 g) was added. Photographs were taken at each time interval of 20 s. At the end of photographing, the plate was wiped with the GS to achieve the complete removal.
Fig. 2. SEM images of GS at lower (a) and higher (b) magnifications.
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weighted again (m). The adsorption capacity (q) was calculated as following equation:
GO sheets were reduced by glucose via hydrothermal treatment. During the reduction, graphene sheets lost the dispersibility in aqueous system and formed 3D porous structure. According to the SEM images (Fig. 2), GS had large pores (∼100 m) and the graphene sheets cross-linked. Graphene sheets were very large (∼200 m) when comparing to starting GO sheets (∼1 m). This suggesting that graphene sheets assembled to form larger sheets
and then cross-linked to produce 3D porous structure. In the contrast, the SEM image of GO was generally flat (Fig. S1a). Then we cut some GS samples and sonicated them in water for TEM investigation. Under TEM, the graphene sheets showed more wrinkles and folds (Fig. 3) than GO (Fig. S1b). No carbon particle was found on the graphene sheets of GS. This was consistent with the literature results [27–30]. For example, Ji et al. [27] hydrothermally reduced GO by sugars and no apparent particulate was presented on graphene sheets under TEM. Martin-Jimeno et al. [28] used very high glucose: GO ratio and observed the form of carbon layers on graphene sheets rather than spheres. In the contrast, pure glucose formed particles during the hydrothermal treatment. The porous structure and hydrophobic nature of GS made it floating on water easily. This might benefit the applications in oil adsorption, because oils usually float on water. XRD was adopted to reveal the crystalline of GS (Fig. 4). There was no peak found in the XRD spectrum of GO (Fig. S2a). After the reduction, a broad and weak peak was observed at 2 of 24.6◦ for GS. It migrated to lower 2 angles comparing to graphite (2 of 26.6◦ ). This suggested that the interlayer spacing of graphene layers (002) was larger in GS (0.36 nm) than in graphite (0.33 nm). In the IR spectrum, a broad band at 3436 cm−1 was assigned to COOH/ OH (Fig. 5a). The tiny peak at 2920 cm−1 corresponded to CH, which should be the remnant glucose residues. The peak at 1630 cm−1 was attributed to C O. The peak at 1030 cm−1 was
Fig. 3. TEM images of GS at lower (a) and higher (b) magnifications.
Fig. 4. XRD spectra of GS (a) and graphite (b).
q=
m − m0 m0
(1)
The recycling GS, upon the adsorption of dodecane, GS samples were squeezed and then dried by natural evaporation. The dried GS was used to adsorb dodecane again and the adsorption capacity was measured as described above. The recycling was repeated up to 10 cycles. To demonstrate the removal of dodecane by burning, a piece of GS was placed in a glass place and over dosed dodecane was added. The photographs were taken before and after firing. To measure the size of GS, a rule was place under the plate. 3. Results and discussion 3.1. Characterization of GS
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Fig. 5. IR (a), C1s XPS (b) and Raman (c) spectra of GS.
assigned to C O. The IR spectrum of GS was similar to that of GO. Such phenomenon was also observed in Ji et al.’s study [27]. The remnant oxygen was quantified by XPS. The atomic ratio of oxygen was 14.6%. The value was 33.5% for GO. Thus, efficient reduction was achieved according to XPS. The remnant oxygen of GS was reasonable and consistent with the literature results, in which chemical reduction was unable to remove all the oxygen atoms [31–33]. In
addition, the C1s XPS was recorded to reveal the chemical states of carbon atoms (Fig. 5b). The components of GS were C C (61.8%), C O (34.1%) and C O (4.13%). In the contrast, the components of GO were C C (43.7%), C O (49.3%) and C O (7.0%) (Fig. S2b). The Raman spectrum of GS showed similar intensities of G-band and D-band (Fig. 5c), while the intensity of G-band was weaker than that of D-band in GO (Fig. S2c).
Fig. 6. Adsorption of dodecane (dyed with Sudan red 5B) by GS. The photographs were taken before (a) and after adding of GS at time intervals of 20 s (b–h).
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Fig. 7. Adsorption of dodecane (dyed with Sudan red 5B) by GS in simulated seawater. (a) Dodecane in seawater, (b) at the beginning of adding GS, (c) at 8 min post the addition of GS. Table 1 Adsorption capacity of various adsorbents for oils and solvents. Adsorbent
Oil or solvent
Capacity (g/g)
Ref.
Corn stalk Polypropylene Modified polyurethane sponge Butyl rubber Graphene foam Carbon foam Graphene coated polyurethane Carbon nanofiber hydrogel Graphene sponge Polystyrene fibers Graphene-carbon nanotube GS
Gas oil Fuel oil Dodecane Crude oil Oil Wash oil Acetone Diesel oil Castor oil Silicon oil Toluene Crude oil
8 15.7 18 23 27.1 28.4 32 40 75 81.4 130 29.3
[38] [39] [40] [39] [24] [41] [20] [42] [15] [43] [44] This study
To demonstrate the removal of oil by GS, dodecane was stained by Sudan red 5B and placed in a glass plate for adsorption (Fig. 6a). Then, a piece of GS was added and photographed at time intervals of 20 s (Fig. 6b–f). The remnant dodecane was wiped to reach complete removal (Fig. 6g, h). The results clearly indicated that GS was capable for oil adsorption. Further, many oil leakages occur in sea, thus, we evaluated the adsorption of dodecane in simulated seawater (Fig. 7). After 8 min
adsorption, the dodecane was completely removed. The side view of the beaker was provided in Fig. S3. The adsorption was slower than in pure liquid form, because when the contacting dodecane was adsorbed, water would fill the gap between GS and dodecane. We used a sucker to drive GS toward dodecane and the adsorption was slowed down. During the adsorption experiment, GS floated steadily on the simulated seawater. It would benefit the practical applications, where GS can be collected easily after the adsorption. Then, the adsorption capacities of GS for various organic solvents and oils were quantified. As shown in Fig. 8, GS was capable in removal diverse pollutants. The adsorption capacities were in the range of 23–35 g/g. The ultrahigh adsorption performance suggested that the adsorption was more like accommodation of oils in the pores of GS. The adsorption mechanism should be that the pores were hydrophobic and the oils filled the pores. In particular, the adsorption capacity for crude oil was 29.3 g/g, indicating
Fig. 9. Contact angle of GS.
Fig. 10. Regeneration of GS after the adsorption of dodecane.
Fig. 8. Adsorption capacity of GS for organic solvents and oils.
3.2. Adsorption of oil and organic solvents on GS
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Fig. 11. Removal of dodecane from GS by burning. (a) Before firing, (b, c) during the burning, (d) after the burning.
that GS could be used for oil leakage treatment. The adsorption for machine oil had the highest capacity of 35.5 g/g. Due to the remnant oxygen atoms, GS was also capable for the adsorption of polar solvents, such as ethanol, acetone, and so on. This was due to the very weak hydrophilicity of GS. As shown in Fig. 9, the contact angle of GS sample was 83◦ , smaller than that of well reduced graphene [34]. It should be noted that although the adsorption capacities of our GS samples were lower than those of other GS from hydrothermal methods [15,16], the performance of our samples was higher than other high-performance adsorbents, such as alumina [35], polymers [36], and natural products [37]. The comparison was listed in Table 1. Considering the environmental friendly preparation [15,16], our GS would found practical applications in oil removal. For practical applications, the recycling of GS is very important for reducing the operating cost. The regeneration of GS was achieved by squeezing and evaporating. After 2 cycles, the adsorption capacity retained around 80% of the raw GS (Fig. 10). At 10 cycles, the adsorption capacity was 77% of the raw GS. In addition, GS could be recycled by burning on fire. As shown in Fig. 11, GS did not change much before and after the burning of dodecane. The Raman and IR spectra of GS before and after the burning did not change much, confirming the unchanged properties of GS during the burning (Fig. S4). The phenomenon should be attributed to the good thermo conductivity of GS. The regeneration by burning seemed to be even better than evaporating method. However, burning led to the waste of oil and would pollute the air. Therefore, we recommended the squeezing and evaporating to be better regenerating method.
4. Conclusion GS was prepared via one-pot hydrothermal reduction for oil removal, where environmental friendly glucose was used as the reducing reagent. Hydrophobic graphene sheets formed 3D porous structure that could accommodate organic solvents and oils. GS showed competitive adsorption capacities to other high performance adsorbents for oil removal. The facile regeneration of GS made it very promising in practical applications. It is hoped that our study would stimulate more interest on the graphene based environmental nanotechnologies.
Acknowledgments We acknowledge financial support from the Science and Technology Department of Sichuan Province (No. 2013FZ0060), the China Natural Science Foundation (No. 201307101), and the Innovation Scientific Research Program for Graduates in Southwest University for Nationalities (No. CX2016SZ024).
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